Old Earth Ministries Online Geology Curriculum

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Geology - Chapter 12: Weathering

Weathering
is the process of breaking down rocks, soils and their minerals through
direct contact with the atmosphere. Weathering occurs in situ,
or 'without movement', and thus should not to be confused with erosion,
which involves the movement and disintegration of rocks and minerals by
processes such as water, wind, ice, hail and gravity.

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Two main classifications of weathering processes exist. Mechanical or
physical weathering involves the breakdown of rocks and soils through direct
contact with atmospheric conditions such as heat, water, ice and pressure.
The second classification, chemical weathering, involves the direct effect
of atmospheric chemicals, or biologically produced chemicals (also known as
biological weathering), in the breakdown of rocks, soils and minerals.
The materials left over after the rock breaks down combined with
organic material creates soil. The mineral content of the soil is determined
by the parent material, thus a soil derived from a single rock type can
often be deficient in one or more minerals for good fertility, while a soil
weathered from a mix of rock types (as in glacial, eolian or alluvial
sediments) often makes more fertile soil.

Physical (mechanical) weathering

Mechanical weathering is a cause of the
disintegration of rocks. Most of the times it produces smaller angular
fragments (like scree), as compared to chemical weathering. However,
chemical and physical weathering often go hand in hand. For example, cracks
exploited by mechanical weathering will increase the surface area exposed to
chemical action. Furthermore, the chemical action at minerals in cracks can
aid the disintegration process.

Thermal Expansion

Thermal expansion, also known as onion-skin
weathering, exfoliation or thermal shock, often occurs in areas, like
deserts, where there is a large temperature range on a daily basis. The
temperatures soar high in the day, while dipping greatly at night. As the
rock heats up and expands by day, and cools and contracts by night, stress
is often exerted on the outer layers. The stress causes the peeling off of
the outer layers of rocks in thin sheets. Though this is caused mainly by
temperature changes, thermal expansion is enhanced by the presence of
moisture.

Freeze Thaw Weathering

Freeze thaw weathering can also be called
frost shattering. This type of weathering is common in mountain areas where
the temperature is around freezing point (see picture). Frost induced
weathering, although often attributed to the expansion of freezing water
captured in cracks, is generally independent of the water-to-ice expansion.
It has long been known that moist soils expand or
frost
heave upon freezing as a result of water migrating along from unfrozen
areas via thin films to collect at growing ice lenses. This same phenomena
occurs within pore spaces of rocks. They grow larger as they attract liquid
water from the surrounding pores. The ice crystal growth weakens the rocks
which, in time, break up. Intermolecular forces acting between the mineral
surfaces, ice, and water sustain these unfrozen films which transport
moisture and generate pressure between mineral surfaces as the lens
aggregates. Experiments show that chalk, sandstone and limestone do not
fracture at the nominal freezing temperature of water of slightly below 0°C,
even when cycled or held at low temperature for extended periods, as one
would expect if weathering resulted from the expansion of water as froze.
For the more porous types of rocks, the temperature range critical for
rapid, ice-lens-induced fracture is -3 to -6°C, significantly below freezing
temperatures.
Freeze induced weathering action occurs mainly in environments where
there is a lot of moisture, and temperatures frequently fluctuate above and
below freezing point—that is, mainly alpine and areas around glaciers. An
example of rocks susceptible to frost action is chalk, which has many pore
spaces for the growth of ice crystals.

Frost (Ice) Wedging

Frost action, sometimes known as ice crystal growth, ice
wedging, frost wedging or freeze-thaw occurs when water in
cracks and joints of rocks freeze and expand. In the expansion, it was
argued that since expanding water can exert pressures up to 21
megapascals (MPa) (2100 kgf/cm²) at −22 °C. This pressure is often
higher than the resistance of most rocks and causes the rock to shatter.
When water that has entered the joints freezes, the ice formed strains
the walls of the joints and causes the joints to deepen and widen. This is
because the volume of water expands by 10% when it freezes.
When the ice thaws, water can flow further into the rock. When the
temperature drops below freezing point and the water freezes again, the ice
enlarges the joints further.
Repeated freeze-thaw action weakens the rocks which, over time, break
up along the joints into angular pieces. The angular rock fragments gather
at the foot of the slope to form a talus slope (or
scree
slope). The splitting of rocks along the joints into blocks is called block
disintegration. The blocks of rocks that are detached are of various shapes
depending on rock structure.

Pressure Release

In pressure release, also known as unloading,
overlying materials (not necessarily rocks) are removed (by erosion, or
other processes), which causes underlying rocks to expand and fracture
parallel to the surface . Often the overlying material is heavy, and the
underlying rocks experience high pressure under them, for example, a moving
glacier. Pressure release may also cause exfoliation to occur.
Intrusive igneous rocks (e.g. granite) are formed deep beneath the
earth's surface. They are under tremendous pressure because of the overlying
rock material. When erosion removes the overlying rock material, these
intrusive rocks are exposed and the pressure
on them is released. The outer parts of the rocks then tend to expand. The
expansion sets up stresses which cause fractures parallel to the rock
surface to form(see picture). Over time, sheets of rock break away from the
exposed rocks along the fractures. Pressure release is also known as
"exfoliation" or "sheeting";
these processes result in batholiths and granite domes.

Hydraulic action

Hydraulic action is when water (generally
from powerful waves) rushes into cracks in the rockface rapidly. This traps
a layer of air at the bottom of the crack, compressing it and weakening the
rock. When the wave retreats, the trapped air is suddenly released with
explosive force. The explosive release of highly pressurized air cracks away
fragments at the rockface and widens the crack itself, worsening the process
so more air is trapped on the next wave. This progressive system of positive
feedback can damage cliffs greatly and cause rapid weathering.

Salt-Crystal Growth (haloclasty)

Salt crystallization or otherwise known as
haloclasty
causes disintegration of rocks when saline solutions seep into cracks and
joints in the rocks and evaporate, leaving salt crystals behind.. As
the salt crystals continually build up, they can exert more and more force
upon the rock, until the rock fractures.
Salt crystallization may also take place when solutions decompose rocks
(for example, limestone and chalk) to form salt solutions of sodium sulfate
or sodium carbonate, of which the moisture evaporates to form their
respective salt crystals.
The salts which have proved most effective in disintegrating rocks are
sodium sulfate, magnesium sulfate, and calcium chloride. Some of these salts
can expand up to three times or even more.
It is normally associated with arid climates where strong heating
causes strong evaporation and therefore salt crystallization. It is also
common along coasts. An example of salt weathering can be seen in the
honeycombed stones in sea walls.

Biotic Weathering

Living organisms may contribute to mechanical weathering (as well as
chemical weathering, see 'biological' weathering below). Lichens and mosses
grow on essentially bare rock surfaces and create a more humid chemical
microenvironment. The attachment of these organisms to the rock surface
enhances physical as well as chemical breakdown of the surface microlayer of
the rock. On a larger scale seedlings sprouting in a crevice and plant roots
exert physical pressure as well as providing a pathway for water and
chemical infiltration. Burrowing animals and insects disturb the soil layer
adjacent to the bedrock surface thus further increasing water and acid
infiltration and exposure to oxidation processes.
Another well known example of animal-caused biotic weathering is by the bivalve mollusk known as a
Piddock.
These animals, found 'boring' into carboniferous rocks, such as the
limestone cliffs of Flamborough Head, bore themselves further into the
cliff-face.

Chemical Weathering

Chemical weathering involves the change in
the composition of rock, often leading to a 'break down' in its form. The
discussion below includes the chemical formulas, for those who want to study
them in depth. Specific formulas will not be testable material, although
the tests may include the common names of the minerals.

Dissolution

Dissolution is the process of dissolving a
solid into a liquid, usually with the help of an acid. Rainfall is
naturally slightly acidic because atmospheric carbon dioxide dissolves in
the rainwater producing weak carbonic acid. In unpolluted environments, the
rainfall pH is around 5.6. Acid rain occurs when gases such as sulfur
dioxide and nitrogen oxides are present in the atmosphere. These oxides
react in the rain water to produce stronger acids and can lower the pH to
4.5 or even 4.0. Sulfur dioxide, SO2, comes from volcanic
eruptions or from fossil fuels, can become sulfuric acid within rainwater,
which can cause solution weathering to the rocks on which it falls.
One of the most well-known solution weathering processes is
carbonation, the process in which atmospheric carbon dioxide
leads to solution weathering. Carbonation occurs on rocks which contain
calcium carbonate such as limestone and chalk. This takes place when rain
combines with carbon dioxide or an organic acid to form a weak carbonic acid
which reacts with calcium carbonate (the limestone) and forms calcium
bicarbonate. This process speeds up with a decrease in temperature and
therefore is a large feature of glacial weathering.

Hydration is a form of chemical weathering
that involves the rigid attachment of H+ and OH- ions to the atoms and
molecules of a mineral.
When rock minerals take up water, the increased volume creates physical
stresses within the rock. For example iron oxides are converted to iron
hydroxides and the hydration of anhydrite forms gypsum.

Hydrolysis

Hydrolysis is a chemical weathering process affecting
silicate minerals. In such reactions, pure water ionizes slightly and reacts
with silicate minerals. An example reaction:

This reaction results in complete dissolution of the original mineral,
assuming enough water is available to drive the reaction. However, the above
reaction is to a degree deceptive because pure water rarely acts as a H+
donor. Carbon dioxide, though, dissolves readily in water forming a weak
acid and H+ donor.

This hydrolysis reaction is much more common. Carbonic acid is consumed
by silicate weathering, resulting in more alkaline solutions because of the
bicarbonate. This is an important reaction in controlling the amount of CO2
in the atmosphere and can affect climate.
Aluminosilicates when subjected to the hydrolosis reaction produce a
secondary mineral rather than simply releasing cations.

Within the weathering environment chemical
oxidation of a variety of metals occurs. The most commonly
observed is the oxidation of Fe2+ (iron) and combination with
oxygen and water to form Fe3+ hydroxides and oxides such as
goethite, limonite, and hematite. This gives the affected rocks a
reddish-brown coloration on the surface which crumbles easily and weakens
the rock. This process is better known as 'rusting'.

Biological

A number of plants and animals may create chemical weathering through
release of acidic compounds. The most common form of biological weathering is the release of chelating
compounds, that is acids, by trees so as to break down elements such as
aluminum and iron in the soils beneath them. Once broken down, such elements
are more easily washed away by rainwater. This process exists as metals such
as iron can be toxic and hinder the a tree's growth. Extreme release of
chelating compounds can easily affect surrounding rocks and soils, and may
lead to
podsolisation of soils.

Building Weathering

Buildings made of limestone are particularly susceptible to weathering.
Weeds grow almost anywhere without many problems. They can sometimes
germinate in the gutters of buildings where they have been transported to by
the wind. As they proceed to grow they plant their roots down into the rock
that the building is made up of forcing their way further down. This causes
the rock to exfoliate over a long time, small fragments crumbling away now
and then. Statues and ornamental features can be badly damaged by
weathering, especially in areas severely affected by acid rain which is
caused by pollutants put into the air.

The process of jointing greatly increases the amount of surface space
exposed to weathering. To visualize this process, consider the Rubik's
Cube (picture at right).

Imagine that the white surface is the earth's surface. If this cube
were a solid block of rock, then only the white surface would be exposed to
weathering. Let's imagine that the entire cube is 3 meters across.
Therefore the entire surface area exposed to weathering is 9 cubic meters.

If joints develop from weathering, so that each
colored face is exposed (the outer surfaces), then the total surface now
exposed to weathering would be six times greater, or 54 cubic meters.
If further joints develop, one meter apart, breaking the rock into three
sections across and three sections wide, and three sections deep, we now
have 27 smaller cubes, with all their surfaces exposed to weathering.
This would increase the total surface area exposed to weathering to 162
square meters.

The joints in the rock provide a system of channels through which water can
move, leading to more mechanical and chemical weathering. This process
can occur on the surface, or even hundreds of meters beneath the earth's
surface. The image at right shows an aerial photo of a large rock body
at Arches National Park that is cut by many joints.

The Products of Weathering

If you have ever examined dirt, then you have examined the products of
weathering. As
rocks weather, they form what is known as regolith,
which is the loose soil that covers the earth's bedrock. You can see
the transition of rock to regolith in road cuts or stream valleys. The
image at left shows the regolith on top of bedrock. The fractured
rocks at the bottom of the regolith are part of the regolith. As
joints (fractures) progress downward, the top of the bedrock erodes, and the
base of the regolith moves deeper.

The upper layer of the regolith is known as soil. Soil is further
divided into divisions known as horizons, which
are distinguished by composition, color, and texture.

Rocks typically weather into rounded surfaces. This is the result of
weathering occurring on all sides at one time. This tendency is known
as
spheroidal weathering.

A special type of spheroidal weathering is known as
exfoliation, which was discussed above.

Climate and Weathering

Weathering is a direct result of the climate. The climate will
determine the rate and type of weathering that occurs to a rock. The
greatest weathering agent is water. The amount of precipitation in an
area determines the amount of weathering that can occur, although other
factors also play a part, such as the intensity of the rain, rate of
evaporation, water drainage, and water infiltration into the soil.

Tropical, humid environments produce the thickest layers of soil.
Because of the high precipitation rate, and high temperatures, chemical
weathering can occur rapidly, developing soils with depths of greater than
70 meters. In more arid regions, the soil may be completely absent,
with the bedrock exposed. In polar climates, the soils is very thin,
because it is too cold for much chemical weathering to occur.

The rate of weathering depends upon three main factors.

1. The minerals being weathered and
their resistance to weathering

2. The climate

3. The amount of surface area exposed

Minerals can have varying degrees of resistance to weathering.
Typically, minerals that crystallize at a high temperature weather the
easiest.

Climate makes a large impact. For example, in Hawaii, fresh lava flows
will weather enough in a few years, and can support vegetation. Lava
flows with the same composition in other, less humid environments, have
remained unweathered since the ice ages ended over 11,000 years ago.

Scientists can use known rock surfaces to measure weathering rates.
For example,
buildings, tombstones, and monuments have known dates of construction, and
their rock surfaces can be examined for erosion. A great example of
this is the pyramids of Egypt. The pyramid at right has visible debris
on each step, which comes from the erosion of rocks that make up the
pyramid.

Today you will
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quiz, continue your research project, if necessary.